Let platform manifests go only on a confirmed collect, and copy again what a sweep took

An unrecorded index or a copy in progress can name a platform the records do not see, so
only a person's collect, after its dry run, takes an index's platforms, and only once every
kept index of each repository it touches was read. A copy missing a platform is copied
again, and a copy a build holds again is no longer recorded as collected (review of #144).

internal/builder/mirror_test.go

@@ -117,6 +117,13 @@ func (m *theMeshsRegistry) handler() http.Handler {
 			} else {
 				w.WriteHeader(http.StatusNotFound)
 			}
+		case r.Method == http.MethodGet && strings.Contains(r.URL.Path, "/manifests/"):
+			body, ok := m.manifests[r.URL.Path[strings.LastIndex(r.URL.Path, "/")+1:]]
+			if !ok {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			_, _ = w.Write(body)
 		case r.Method == http.MethodHead && strings.Contains(r.URL.Path, "/blobs/"):
 			if _, ok := m.blobs[r.URL.Path[strings.LastIndex(r.URL.Path, "/")+1:]]; ok {
 				w.WriteHeader(http.StatusOK)